1. Field
The present invention relates to a control system for an internal combustion engine having a valve operating characteristic varying mechanism for continuously varying a lift amount of the intake valve(s), and particularly to a control system having a failure diagnosis function for the valve operating characteristic varying mechanism.
2. Description of the Related Art
Japanese Patent Publication No. 3463463 (JP'463) discloses a diagnosis apparatus for diagnosing an abnormality of a plurality of sensors for detecting a same object parameter, based on the outputs of the plurality of sensors. According to this apparatus, an offset is prepared for at least one of the minimum value side and the maximum value side of the output characteristic of the sensor, and the reference points of the sensor outputs are corrected by adjusting the offset(s) of each sensor. With this correction, incorrect diagnosis due to differences among the reference points of the plurality of sensors is prevented.
If the apparatus shown in the above-described JP'463 is applied to a valve operating characteristic varying mechanism which changes a lift amount of the intake valve(s) by actuating a control shaft with a motor connected to the control shaft via an actuating force transmission mechanism, the following effect is obtained. That is, if a plurality of sensors for detecting a rotational angle of the control shaft are provided, for example, reliability of the detected rotational angle can be improved and a failure diagnosis of the plurality of sensors can be performed. However, since the plurality of sensors detects a same object parameter, i.e., a rotational angle of the control shaft, it is impossible to diagnose a failure of the actuating force transmission mechanism for transmitting the actuating force to the control shaft.